Polyvinyl alcohol polymers are water soluble and are widely used in various industrial fields. In particular, polyvinyl alcohol polymers are often used as a dispersion stabilizer for suspension polymerization or emulsion polymerization, a dispersant for inorganic substances or organic powders, or the like, because of their surface activity. In the past, various attempts have been made for further enhancing the water solubility or surface activity of the polyvinyl alcohol polymers in such applications. For example, JP 61 (1986)-41924 B discloses a polyvinyl alcohol polymer having a thioalkyl group with 1 to 18 carbon atoms at one end of its main chain, and JP 61 (1986)-254238 A discloses that the above polymer has excellent surface activity and is useful as a dispersion stabilizer for coal-water slurry. Furthermore, JP 10 (1998)-168128 A discloses a polyvinyl alcohol polymer having an ionic group at one end thereof, and describes that it is useful for suspension polymerization of vinyl compounds. However, the above-mentioned polyvinyl alcohol polymers have room for improvement in water solubility, though they are excellent in surface activity.